1. Field of the Invention
The present invention relates to a hands-free circuitry for use in a mobile telephone system, etc. which prevents howling caused by acoustic coupling between a speaker and a microphone of a telephone set.
2. Description of the Related Art
In a mobile telephone system, etc. in which two parties talk with each other on a duplex communication basis with use of a speaker and a microphone, a so-called hands-free control circuit is used for the purpose of preventing howling caused by acoustic coupling between the speaker and microphone.
For a hands-free control circuit, a voice switch type circuit is typically used in which the level of a signal outputted from a microphone (hereinafter refer to as transmit audio signal) is compared with that of a signal to be inputted to a speaker (hereinafter refer to as receive audio signal), and when the level of the transmit audio signal is higher, the level of the receive audio signal is lowered so that the circuit is in the "transmit mode" and when the level of the receive audio signal is higher, the level of the transmit audio signal is lowered so that the circuit is in the "receive mode".
FIG. 10 shows an example of a hands-free control circuit of the voice switch type which is used in a mobile telephone system.
In FIG. 10, a transmit audio signal outputted from a microphone 5 is transmitted to a transceiver (transmitter) (not shown) through an attenuating circuit 71 and an interface circuit 8 and then transmitted to a telephone network. The attenuating circuit 71, which comprises a resistor and a switch SW connected in parallel to the resistor, performs its attenuating function by turning on and off the switch SW.
On the other hand, a receive audio signal, which is transmitted from the other party and received at the transceiver (receiver) (not shown), is inputted into a speaker 4 through an interface circuit 1, an attenuating circuit 2 and an amplifier 3. The attenuating circuit 2 functions to adjust the sound volume of the speaker 4.
A comparing circuit 12 compares the level of the transmit audio signal transmitted from the microphone 5 via the amplifier 6 and detected at a transmit audio rectifier 11 with the level of the receive audio signal transmitted from the other party and detected at a receive audio rectifier 10, and outputs a logic signal corresponding to its comparison result.
A logic control circuit 13 judges, on the basis of an output of the comparing circuit 12, whether or not the hands-free control circuit is in the transmit mode or in the receive mode, and controls the attenuating circuit 7 or 2 according to its judgement result under the control of a command received from a CPU (not shown).
When the logic control circuit 13 judges that the hands-free control circuit is in the transmit mode, the receive audio signal is attenuated; while, when the logic control circuit 13 judges that the circuit in the receive mode, the transmit audio signal is attenuated, as described above.
FIG. 11 shows attenuation characteristics of transmit and receive levels in the transmit and receive modes when the above control is carried out.
As shown in FIG. 11, in the prior art hands-free control circuit, the receive audio signal and the transmit audio signal are selectively attenuated depending on the transmit or receive mode with a constant attenuation factor.
Because the attenuation factor is constant, when the mode is changed from the receive mode to the transmit mode under such a condition that the receive audio volume is lowered by user's manual operation to a speaker volume, the receive audio volume will be further attenuated according to the constant attenuation factor.
When the receive audio volume is low, a howling margin is sufficiently high with a small howling possibility. However, further attenuating the receive audio volume based on the constant attenuation factor is undesirable because the receive audio volume is attenuated down to an unnecessary low level.
As a result, the other party's voice becomes hardly audible especially when the receive audio volume level is low, and therefore a so-called simplex communication is likely to be incurred.
As described above, in the conventional hands-free control circuit, the factor of attenuation when attenuating the receive audio signal or the transmit audio signal according to the transmit or receive mode is always constant. Thus, the conventional hands-free control circuit has a problem that, when switching is carried out from the receive mode to the transmit mode with the speaker volume set to a low level, the receive audio volume is attenuated too far. Therefore, the other party's voice becomes hardly audible, that is, the simplex communication is incurred.